(1) Field of the Invention
The present invention relates to a catalyst deterioration suppressing apparatus and method for suppressing deterioration of an exhaust purifying catalyst that purifies toxic substances in exhaust gas emitted from an engine.
(2) Description of Related Art
Generally, as the temperature of the exhaust gas increases and the atmosphere of the exhaust gas becomes more oxidized (the air-fuel ratio becomes leaner), an exhaust purifying catalyst (hereinafter referred to as “the catalyst”) disposed in an exhaust passage of an engine for purifying toxic substances in exhaust gas is more likely to deteriorate by heat due to sintering (the phenomenon in which particles held on a carrier are agglomerated to increase the particle diameter) or the like. Thus, the temperature which the catalyst can resist is lower in oxidized atmosphere than in oxygen reduced atmosphere (where the air-fuel ratio is rich).
Therefore, to suppress such a heat deterioration of the catalyst, it is necessary to surely avoid the situation where the catalyst temperature becomes high and the catalyst is exposed to oxidized atmosphere.
In recent years, to reduce CO2, which leads to reduction in fuel consumption, a vehicle has been put into practical use which is equipped with a fuel cut device that is capable of temporarily shutting off supply of fuel (i.e. cutting fuel) to all or part of cylinders disposed in an engine when the vehicle is brought into a predetermined operative state, e.g. a decelerating state while running.
However, when fuel is cut as mentioned above, only air is emitted from the cylinder or cylinders for which fuel has been cut, the exhaust air-fuel ratio is likely to be lean.
Therefore, in the case of the above described engine, a catalytic converter is exposed to oxidized atmosphere and the temperature thereof becomes high with a high frequency when fuel is cut.
To address this problem, the technique has been disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 55-137339 in which fuel is prohibited from being cut when the catalyst temperature is high. Also, the technique has been disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 8-144814 has been disclosed in which fuel is prohibited from being cut when the catalyst bed temperature estimated based on the intake air volume is high, or fuel is prohibited from being cut based on the engine speed and the engine load.
Generally, when fuel injection is controlled, the fuel injection quantity is feedback controlled so that the actual air-fuel ratio can be equal to the target air-fuel ratio.
However, when fuel is prohibited from being cut as described above, the driver does not depress an accel pedal, and hence an engine is operating in an operative range where the intake air volume and the opening time (injector pulse width) of a fuel injection valve are extremely small. As shown in FIG. 6, in this operative range, the linearity of the fuel injection quantity with respect to the injector pulse width deteriorates. If the fuel injection quantity is feedback controlled in this operative range, it is impossible to provide proper control because the integration correction value for feedback-control cannot be stable as described in Japanese Patent Publication No. 6-63468, or for other reasons. This leads to deterioration of exhaust gas, and makes it impossible to control the air-fuel ratio in a stable manner. If the air-fuel ratio becomes lean as a result, the catalyst is brought into oxidized atmosphere, accelerating deterioration of the catalyst.
It should be noted that, the intake air volume is also small while the engine is idling, and on this occasion, positive torque, enough to rotate the engine against friction in engine revolution, is required. On the other hand, positive torque is not required in decelerating, and if positive torque is generated, a sense of deceleration cannot be obtained since the engine is rotating freely without decelerating. For this reason, when fuel is prohibited from being cut in decelerating, it is necessary to generate such a slight torque as to obtain a sense of deceleration. Specifically, engine torque in idling is greater than torque generated when fuel is prohibited from being cut in decelerating.
Therefore, the intake air volume and the fuel injection quantity in idling are greater than those while fuel is prohibited from being cut in decelerating. Thus, while the engine is idling, there is no problem with the linearity since the intake air volume and the fuel injection quantity are greater than those while fuel is prohibited from being cut in decelerating.
It is therefore an object of the present invention to provide a catalyst deterioration suppressing apparatus capable of suppressing deterioration of the catalyst by controlling the air-fuel ratio in a stable manner even in an operative range where the intake air volume is extremely small and the linearity of the fuel injection quantity deteriorates while fuel is being prohibited from being cut.